Unvendor CLI11 since it is now shipped in all supported platforms

[azeey]

CLI11 was vendored in #2 because it wasn't available in Ubuntu Focal. We can now unvendor it since it's available everywhere we support.

Platform	version	Link
Ubuntu Noble	2.4.1	https://packages.ubuntu.com/noble/libcli11-dev
Homebrew	2.4.2	https://formulae.brew.sh/formula/cli11
Windows vcpkg 2022 snapshot	2.1.2	https://github.com/microsoft/vcpkg/blob/2022.02.23/ports/cli11/vcpkg.json
Windows (conda)	2.4.1	https://anaconda.org/conda-forge/cli11

[traversaro]

In theory it should be sufficient to set GZ_UTILS_VENDOR_CLI11 to OFF

gz-utils/CMakeLists.txt

Lines 25 to 28 in 6d794ba

	option(
	GZ_UTILS_VENDOR_CLI11
	"If true, use the vendored version of CLI11, otherwise use an external one"
	true)

.

[scpeters]

In theory it should be sufficient to set GZ_UTILS_VENDOR_CLI11 to OFF

gz-utils/CMakeLists.txt

Lines 25 to 28 in 6d794ba

	option(
	GZ_UTILS_VENDOR_CLI11
	"If true, use the vendored version of CLI11, otherwise use an external one"
	true)

.

trying this in #167

[scpeters]

this defaults to OFF in Jetty, and we can remove the vendoring in Gazebo K

[scpeters]

re-enabling vendoring by default in #178 as a temporary workaround for gazebosim/gz-sim#2918

[scpeters]

unvendored again in #179

[scpeters]

actually leave open until we remove the vendored copy in Gazebo K